A rocking chair shape archwire was first developed by hand by clinicians in round cross-section, stainless steel archwires to correct or negate the oral forces necessary to produce the curve of spee. Another name used for the reverse curve of spee arch is the "M" arch, named after Dr. Margolias of Boston University. The M arch was used by clinicians from the 1960's throughout the early 1980's for early treatment of typical malocclusions with deep overbites often with wide diastemas mixed dentition between the maxillary incisors, and generally for retraction and intrusion of same.
Each M archwire was custom handcrafted on an individual patient basis since there were none commercially available in a standardized configuration. Thus, clinicians were involved in time consuming hand fabrication of an M arch for each patient. In an article entitled "The M Arch", by Jerome L. Blafer, D.D.S., M arch is described as basically a wire with an exaggerated curve of spee and exaggerated molar contraction bends. It could be used in conjunction with coil springs on the arch to complete the basic force system. Further, clinicians using the M arch treatment necessarily had to see their patients at approximately 3-4 week intervals, at which time the arch could be reformed, i.e., new force moments could be incorporated into the archwire so that there would be new forces imparted to continue the movement of the teeth to a desired position. Over the course of a few weeks, the round stainless steel M archwire would gradually loose its ability to impart forces to the teeth so that at the end of the 3-4 week period, the archwire had to be reshaped to impart new forces. Also, the M archwire generally required use of elastics, looping devices, and head gear to enhance the movement of teeth to the desired position.
Other prior art appliances used to position teeth included curved archwires, archwires with loops, and archwires having a generally U-shaped configuration. Such archwires are more fully disclosed in U.S. Pat. Nos. 4,268,250, 4,097,993, and 4,197,643. Some of the prior art archwires required closing loops bent into the archwire. Such looped wires had inherently less control in moving teeth as compared to a solid wire. Loss of tip and rotational control adjacent to the loops can occur and arch form and occlusal plane control is not optimum. Loops tend to trap plaque and food debris and can create soft tissue irritation. Activation and adjustment of loop systems, usually at three to four week intervals, can be troublesome. Fabrication, placement and removal of these systems are often time consuming. Re-leveling and root uprighting are often necessary to regain complete control as the finishing treatment stages begin.
The prior art also includes U.S. Pat. No. 4,037,324, which discloses the use of an archwire having a specific chemical composition which wire cause the archwire to return to a preset shape or length after being deformed and then heated. The mechanical memory of the wire will tend to restore the will to a preset shape upon heating in order to level or torque malposed teeth. The prior art wires are formed of a Nitinol alloy which is a known near-stoichiometric alloy of nickel and titanium. The alloy may also include cobalt substituted for nickel on an atom-for-atom basis so that the composition is TiNi.sub.0.935 Co.sub.0.065. Even though the prior art discloses an archwire composition having desirable memory retaining qualities, it does not fully utilize the advantages of such qualities.
The primary object of the present invention is to provide an archwire to move teeth en masse by configuring the archwire so that as it returns to a preset shape, it will provide low, continual forces to the teeth to position them in the desired manner.
It is a further object of the invention to provide a pre-formed archwire having a specific configuration and which is composed of a near-stoichiometric alloy of nickel and titanium so that after attaching it to orthodontic brackets, the archwire will gradually move teeth to their desired position as it returns to its preset shape.
It is an important object of the invention to provide a pre-formed archwire of a specific configuration which incorporates zero plane mechanics so that as the arch is pressed flat at its midline and rocked slowly toward the posterior ends of the archwire, the archwire will remain flat on the surface. This characteristic prevents axial or tortional forces along the long axis of the archwire which would have a tendency to rotate teeth in the facial or lingual direction, i.e. undesirable tipping of roots.
Another object of the invention is to reduce the chair time required by patients in using the prior art devices.
It is a further object of the invention to provide a pre-formed archwire of a specific configuration and composition which the clinician cannot change by reworking or reshaping the archwire.
It is another object of the invention to provide an archwire having reduced frictional forces to permit the brackets to slide easily along the archwire.
It is a still further object of the invention to provide a pre-formed archwire of a specific configuration having a moment-to-force ratio of about 10:1 for closure of spaces without experiencing the adverse aspect of "dumping" or the rotation of a tooth root into an extraction site.
These and other objects will be pointed out and described in further detail hereinafter.